Shareholders are driving changes in corporate policies and disclosures unthinkable a decade ago, on issues ranging from protecting rain forests to human rights. Even the threat of a proxy vote can be enough to bring company executives to the negotiating table.

So far this year, environmental and social issues have accounted for 56% of shareholder proposals, representing a majority for the first time, according to accounting firm Ernst & Young LLP. That is up from about 40% in the previous two years, and means...